The present invention relates to an image forming apparatus such as an electronic copying machine that forms an image using a belt transfer device, and an image forming method.
Recently, an increasing number of belt transfer devices have been used as means for transferring toner images formed on photosensitive drums. When a belt transfer device is used for transferring a toner image onto a transfer medium, the belt transfer apparatus can perform stable separation of the transfer medium from the photosensitive drum and can a have good transfer performance, irrespective of the kind or state of the transfer medium. Moreover, a plurality of times of transfer can easily be performed.
In addition, when a belt transfer device is used as an intermediate transfer device for effecting re-transfer on a transfer medium after completion of toner image transfer on a transfer belt, the degree of freedom in construction is enhanced both for primary transfer on the transfer belt and secondary transfer for the transfer medium. Thus, more various apparatus designs can be made.
However, in particular, in a case where an elastic belt is applied to a transfer member, the peripheral length, thickness and tension of the belt in the initial state will vary with the passing of time. Specifically, the various dimensions and physical properties of the transfer belt, whose design life is about to expire after printing and paper feeding, have varied and in general the speed of movement of the belt has lowered.
On the other hand, a photosensitive drum, which rotates in contact with the transfer belt, rotates at a substantially constant speed from the initial state to the end of the design life. As a result, a difference in speed of movement occurs between the photosensitive drum and the transfer belt. This may lead to defects in images, such as image magnification variation or blurring of images.
Consequently, as the transfer belt approaches the end of the design life, the level of image defectiveness rises. As means for solving these problems, there is a method in which the peripheral speed of the transfer belt is controlled by sensing it by means of a plurality of sensors provided at the position of the transfer belt. Based on differences in sensed times of the sensors, the peripheral speed of the belt is found. There is also a method wherein a rotary-type distance meter is put in contact with the peripheral surface of the belt, and the driving speed is directly read. However, these methods are disadvantageous with respect to an increase in cost, complex construction, lack in quickness, and low precision, and therefore these methods are not easily feasible.
The object of the present invention is to provide an image forming apparatus and an image forming method capable of suppressing, with low cost, defects in images, such as blurring of images or image magnification variation, due to lowering in speed of a transfer belt, without the need to provide sensors for sensing and controlling the variation in speed of the transfer belt.
In order to achieve the object, there is an image forming apparatus which has a belt transfer device using a transfer belt and forms an image, comprising: a drive unit which drives the transfer belt; a measuring section which measures a driving time of the drive unit; an accumulation section which accumulates the driving time measured by the measuring section; and a control section which effects a control to vary a driving speed of the drive unit each time the driving time accumulated in the accumulation section exceeds a predetermined time.
There is also provided an image forming method for an image forming apparatus which has a belt transfer device using a transfer belt and forms an image, comprising: measuring a driving time of a drive unit which drives the transfer belt; accumulating the measured driving time; and effecting a control to accelerate a driving speed of the drive unit by a predetermined amount, each time the accumulated driving time exceeds a predetermined time.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.